1. Field of the Invention
The present invention relates to a contact area sensor having a function as an image sensor. More particularly, the present invention relates to a contact area sensor formed of a plurality of thin film transistors (TFTs) arranged in a matrix form.
2. Description of the Related Art
Recently, solid-state imaging devices having a photoelectric conversion element such as a CCD for reading image signals from light signals of literal or graphic information on paper, picture information, or the like have come into popular use. Such solid-state imaging devices are used as scanners, digital cameras, and the like.
Solid-state imaging devices using a CCD as the photoelectric conversion element include into line sensors and area sensors.
FIG. 4A schematically illustrates a conventional solid-state imaging device using a line sensor having a CCD as the photoelectric conversion element. A light source 401, a line sensor 402, and an object 403 are provided as shown in the figure. The light source 401 and the line sensor 402 scan the object 403 in a scanning direction at a constant speed. Light irradiated from the light source 401 is reflected by the object 403, and the reflected light is read by the line sensor 402 and is converted into image signals.
FIG. 4B schematically illustrates the line sensor 402. The line sensor 402 is formed of a CCD 404 and an optical system 405. The CCD 404 is formed of a plurality of small CCDs (multiple chips). Light which enters the line sensor 402 is focused by the optical system 405 and enters the CCD 404. The light which enters the CCD 404 is converted into image signals as electric signals, and the image is read.
In case of such a conventional solid-state imaging device the light-receiving portion of which is a line sensor, it is necessary to scan the object with the line sensor. However, if the scan is made manually, it is difficult to make the scan in a constant direction. If a machine is used to make the line sensor scan, since a motor and the like for scanning are provided, there are problems that the solid-state imaging device itself can not be made smaller, thinner, and lighter, the power consumption can not be suppressed, and the solid-state imaging device is easily affected by a shock.
Further, in case of such a conventional solid-state imaging device the light-receiving portion of which is a line sensor, the substrate of the CCD is a silicon substrate. This makes it necessary to make light for reading the image of the object enter between the CCD provided in the line sensor and the object. This, in turn, makes it necessary to provide between the CCD provided in the line sensor and the object the optical system 405 for focusing an image to be read, which prevents the solid-state imaging device itself from becoming smaller, thinner, and lighter.
On the other hand, there are also solid-state imaging devices using an area sensor having a CCD as the photoelectric conversion element. Area sensors having a CCD as the photoelectric conversion element are used in video cameras, digital cameras, and the like.
In case of such a solid-state imaging device the light-receiving portion of which is an area sensor, differently from the case of the line sensor, it is not necessary to scan the object with the light-receiving portion. However, since, similarly to the case of the line sensor, the CCD is formed on a silicon substrate, it is necessary to make light for reading the image of the object enter between the CCD provided in the area sensor and the object. This makes it necessary to provide an optical system for focusing an image to be read, which prevents the solid-state imaging device itself from becoming smaller, thinner, and lighter.
Further, since the size of the silicon substrate is limited, when a CCD is used, it is difficult to form an area sensor having a large area.
Further, in a conventional solid-state imaging device, a color image is read by white light passing through color filters of red, green, and blue provided above the respective pixels of the CCD. In other words, three pixels corresponding to red, green, and blue color filters, respectively, as one group, read a color image. Therefore, in this conventional method, the resolution is one third of the actual resolution. For example, in order that the area sensor reads a VGA color image of 640xc3x97480, 640xc3x973xc3x97480 pixels are necessary. In order that the area sensor reads a SVGA color image of 800xc3x97600, 800xc3x973xc3x97600 pixels are necessary. In this way, in the conventional method, the resolution of the read image is only one third of the actual number of the pixels.
As described in the above, it is difficult to make smaller, thinner, and lighter the conventional solid-state imaging device using a CCD as the photoelectric conversion element. Further, the solid-state imaging device is easily affected by a shock.
Accordingly, the present invention is made in view of the above, and an object of the invention is to provide a contact area sensor which is small, thin, and light, and can withstand a shock.
In order to solve the above problems, according to the present invention, a contact area sensor provided with sensor portions arranged in a matrix form on a substrate which transmits light such as of glass or quartz is formed, the sensor portions comprising a thin film transistor (TFT) and a photodiode. This structure makes it unnecessary to provide a complicated optical system for making light for reading the image of an object enter between the area sensor and the object and for focusing the image to be read, which can make the contact area sensor itself smaller, thinner, and lighter.
Further, with regard to the contact area sensor having thin film transistors arranged in a matrix form, differently from the case of a line sensor, it is not necessary to scan the object, which makes the operation easier. Besides, since it is not necessary to provide a motor and the like for scanning, the contact area sensor itself can be made smaller, thinner, and lighter, the power consumption can be suppressed, and the contact area sensor can withstand a shock.
Further, the peripheral circuit can be formed, using thin film transistors, on the same substrate on which the contact area sensor is formed. This makes it possible to make the contact area sensor further smaller, thinner, and lighter, and more portable contact area sensor can be formed.
Further, according to the present invention, lower power consumption is materialized by using LEDs as the light source. Further, by adopting RGB light source switching as the driving method of image reading, the resolution of the image reading is tripled, and highly precise image reading with high image quality is made possible.
Further, the contact area sensor is structured to be capable of dividedly reading a large area image and connecting the divided images on a software. This makes it possible to read a large area which is larger than the readable area by the area sensor.
Still further, the contact area sensor is structured to be capable of organizing the read letters as a data. By tripling the resolution of the image reading, the organization of the read letters as a data can be carried out with more certainty and more accuracy.